Toxoplasma gondii is a parasitic protozoan which is highly prevalent in humans. It is an opportunistic pathogen which can cause severe disease in fetuses and newborn children as well as immuno-compromised adults. Although recent studies have begun to identify the important surface antigens of the growing form of this parasite, little is known of their precise structure and function, and nothing is known about the genes encoding these antigens. The object of the research described in this proposal is to fill these gaps by determining the primary structure of these genes, hence the antigens, and using this information, to construct synthetic peptides in vitro and fusion polypeptides in bacteria which can be tested for immunogenicity. This work will be approached by preparing recombinant DNA molecules containing genomic DNA inserts from T. gondii. Individual recombinants coding for the surface antigens will be identified using specific antibodies and their nucleotide sequence determined by conventional means. From this, the complete amino acid sequence will be determined and used in the prediction of potentially immunogenic regions according to hydrophilicity calculations. Localization of immunogenic sites will also be carried out using fusion polypeptides containing small portions of the antigens as produced in bacterial systems and assaying for antigenicity using monoclonal antibodies. Synthetic oligopeptides and fusion polypeptides corresponding to these regions will be prepared and tested for efficacy in preventing the disease in animal models. These experiments will be the beginning of a long-term commitment to the investigation of the molecular basis of pathogenicity and development of this important parasite.